Looking good Thom, I've seen people with a deliberate buzz cut fade from sideburns to hair that looks nowhere near as good as your natural look does. The short back and sides haircut works great with your beard and I can only assume you're using some kind of ultra precise German microscope technology to analyze the differences you see in your mustache because my eyes see none! At the same time though I believe the key point with all beards is that the beardsman does what makes them happy, grow, trim, shave, whatever feels right to you, although the last one would be a pity